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Full text of "Text Book Of Mechanical Engineering"

1072

Appendix VL

f   i

I !

is 3, and the subsequent unstressing is marked 4. It will be seen7
therefore, that the lag does not increase, and that the final curve
for rapid stressing and unstressing without rest becomes a straight
line joining ab. Evidently hysteresis is not an explanation of
fatigue, and the old fear that continued variation of stress without
rest must ultimately end in rupture, however strong the material,
is certainly not supported by experiment. In fact, the results of
all later endurance tests is to shew that a piece can be made
strong enough to stand unlimited and continuous changes of
stress without breaking.

P. 388. Notes on Testing.óWe shall here give a brief
statement of the directions to be followed in preparing, con-
ducting, and recording a tensile test.

Preliminary:

1.  State the kind of material to be tested.

2.  The   specimen   having   been   prepared,   mark   one-inch

divisions along its length up to 10 ins. and centre-pop
these carefully.

3.   Measure the cross-sectional area at several places and note

the average.

Fixing the Specimen :

4.  Place the specimen between grips in the shackles.

5.  Set the jockey weight to zero, with lever balanced.

6.  Put a slight tension on the specimen in order to tighten

the grips.

7.  Attach the extensometer, setting to the extreme or 10 ins.

marks.

During the Test:

8.  Take extensometer readings at every quarter ton of load,

and tabulate load, extension, remarks. Keeping well
within the elastic limit, these readings may be repeated
from zero.

9.  Entering the plastic stage the extensometer is removed.

Now book loads and extensions at every half ton
Increase : the extensions being measured by dividers and
taken on a well-divided rule. A little, time should be